This is very unexpected UX behaviour though, and not even consistent with other parts of the blueprint planner. Eg if I make a change to absolute co-ordinates and then go to cancel there's the prompt "There are unconfirmed changes" : Back | Discard

Considering there's no version history or diff ...

If you go to overwrite the contents of a blueprint, but then do any action which would normally be conceived as a cancel / not to proceed / no confirmation, the blueprint will still be overwritten.

To reproduce;
1) Have a blueprint with any contents, then use the "Select new contents for this ...

Confirmed, I just witnessed it happen. A bot flew over an ocean with an uncommon biter egg, it hatched in mid air and created an uncommon Jesus biter

I found a deconstruction planner set to never remove landfill would ignore that instruction if the planner is used twice over an area containing concrete placed on landfill. On the second placement it will flag the landfill under the concrete for removal.

To reproduce;

1) Make a custom ...